Would you date a friends sibling?

I went to a party last night that my friend throws every year (christmas), I'm Jewish, but I go anyway, since they're my friends. His sister showed up to the party for the first time in around 8 years, which means the last time I saw her, she was only 12. She's now of course, 20, and I'm 26. She grew up in to a very pretty girl, short, but pretty. But, not only is she too young for me, I would feel wrong dating a friends sibling. So, I told him I would never do anything about me finding her attractive